Ariel stellar characterisation II. Chemical abundances of carbon, nitrogen, and oxygen for 181 planet-host FGK dwarf stars

One of the ultimate goals of the ESA Ariel space mission is to shed light on the formation pathways and evolution of planetary systems in the Solar neighbourhood. Stellar elemental abundances are the cipher key to decode planetary compositional signatures. This makes it imperative to perform a large chemical survey not only of the planets, but their host stars as well. This work is aimed at providing homogeneous chemical abundances for C, N, and O among a sample of 181 stars belonging to Tier 1 of the Ariel mission candidate sample. We applied the spectral synthesis and equivalent width methods to a variety of atomic and molecular indicators ( C i lines at 5052 and 5380.3 O i forbidden line at 6300.3 C _2$ bands at 5128 and 5165 and CN band at 4215 using high-resolution and high signal-to-noise spectra collected with a range of spectrographs. We determined carbon abundances for 180 stars, nitrogen abundances for 105 stars, and oxygen abundances for 89 stars. We analysed the results in the light of the Galactic chemical evolution and in terms of the planetary companion properties. We find that our sample essentially follows standard trends with respect to the metallicity values expected for the C/Fe N/Fe and O/Fe abundance ratios. The proportion between carbon and oxygen abundances (both yields of primary production) is consistent with a constant ratio as O/H increases. Meanwhile, the abundance of nitrogen tends to increase with the increasing of the oxygen abundance, supporting the theoretical assumption of a secondary production of nitrogen. The C/N C/O and N/O abundance ratios are also correlated with Fe/H which might introduce biases in the interpretation of the planetary compositions and formation histories when host stars of different metallicities are compared. Finally, we provide relations that can be used to qualitatively estimate whether the atmospheric composition of planets is enriched (or otherwise) with respect to the host stars.